urinary system. urinary system functions 1) regulate water & electrolyte balance 2) regulate...
TRANSCRIPT
Urinary System
Urinary System Functions
1) Regulate water & electrolyte balance
2) Regulate blood pH
3) Remove waste from blood
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Nephron = functional unit of the urinary system
Filtrate = in capsule
Tubular Fluid = in PCT to DCT
Urine = in collecting duct & beyond
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Cortical Nephrons = about 85%•Most reabsorption & secretion
Juxtamedulary Nephrons = 15%•Produce concentrated urine
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Urine Formation
1) Glomerular Filtration
2) Tubular Reabsorption
& Secretion
3) Water Reabsorption
(Conservation)
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Glomerular Filtration
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Filtration (Endothelial-Capsular) Membrane•Fenestrated Endothelium•Basement Membrane•Filtration Slits
Prevents passage of
cells & most proteins •Plasma = 7% protein•Filtrate = 0.03% protein
Ca++, Fe++, T4 hormone
Bound to proteins
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Blood hydrostatic pressure (BHP) is much higher than normal capillaries
About 60 mm Hg vs 15 mmHg
Because
Afferent arteriole (inlet)
is larger than
efferent arteriole (outlet)
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Capsular hydrostatic pressure (CP)
is about 18 mm Hg vs about 0 mm Hg
for interstitial fluid
Why?
Net Filtration Pressure (NFP)
Glomerular Filtration Rate (GFR) =
amount of filtrate formed by BOTH kidneys per minute
Normal is 12.5 ml for each 1 mm Hg NFP for men & 10.5 ml for women
For NFP = 10 mm Hg
GFR = 125 ml/min
or 180 L/day
What would happen if glomerular hydrostatic pressure dropped
to 50 mm Hg ?
What would be the effect on GFR?
GFR Control1) Autoregulation = self regulation, no
neural or endocrine
Myogenic Mechanism = smooth muscle contracts when stretched
How will smooth muscle of afferent arteriole react if BP decreases? If it increases?
What effect will dilation of afferent arteriole have on GFR? Effect of constriction?
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Juxtaglomerular cells =
Smooth muscle in afferent arteriole, that secretes renin
Macula densa = receptor cells in wall of DCT
Mesangial cells = specialized contractile cells that may also ??
Juxtaglomerular Apparatus
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GFR Control1) Autoregulation: Tubuloglomerular Feedback = Juxtaglomerular apparatus monitors fluid entering DCT and adjusts
GFR to maintain
homeostasis
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If GFR is too high macula densa detects the change & sends chemical signal to Juxtaglomerular cells, which constrict the afferent arteriole, and thereby reduce GFR
Juxtaglomerular Apparatus
GFR Control2) Sympathetic Nervous System
In response to strenuous exercise the sympathetic N.S. redirects blood to heart, brain and skeletal muscle, and away from organs like kidneys by constricting the afferent arterioles
What effect will this have on GFR?
GFR Control3) Renin-Angiotensin Mechanism
When BP drops, sympathetic N.S. triggers release of renin by juxtaglomerular cells
Renin converts a plasma protein, angiotensinogen into angiotensin I
Angiotensin-converting enzyme (ACE) from lungs & kidneys converts angiotensin I into angiotensin II, which is a hormone with many powerful effects
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Angiotensin II Effects
•Systemic Vasoconstriction
•Constricts afferent & efferent arterioles
•Stimulates NaCl & water reabsorption by kidneys
•Stimulates aldosterone release by adrenal cortex
•Stimulates antidiuretic hormone (ADH) release
•Stimulates thirst center in hypothalamus
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Urine Formation
1) Glomerular Filtration
2) Tubular Reabsorption
& Secretion
3) Water Reabsorption
(Conservation)
Proximal Convoluted Tubule (PCT)• Longest & most coiled of portion of tubule• Simple cuboidal epithelium with microvilli• Many mitochondria provide ATP for high
amount of active transport
Tubular Reabsorption = movement of water and solutes from tubular fluid to blood
Tubular Secretion = movement of substances from blood into tubular fluid
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PCT Sodium (Na+) reabsorption = 60 -70% of total through secondary active transport
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PCT Chloride (Cl-) reabsorption = 60 -70% of total mostly by following sodium (electrical attraction)
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PCT Bicarbonate (HCO3-) reabsorption? = most
of total returned to blood by “slight of hand trick” & carbonic anhydrase
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PCT Nutrient (glucose, amino acids) reabsorption = Normally 100% returned via cotransport with Na+
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PCT Nitrogenous waste reabsorption = About half of urea diffuses out of tubular fluid, uric acid diffuses out, NOT creatinine (too large)
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PCT Water reabsorption = 60 -70% of total through osmosis, basically by following solutes, called obligatory water reabsorption, (no control/choice)
Uptake by Peritubular Capillaries
Water = Osmosis
•Build up of interstitial H2O from PCT means greater hydrostatic pressure•Narrow efferent arteriole means low BHP in peritubular capillaries, about 8 mm Hg•Trapped proteins (not filtered with water) means higher colloidal osmotic pressure (COP)
Solutes = Solvent Drag
Water “drags” solutes into capillary
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Transport Maximum (Tm) = maximum rate of reabsorption that is reached when all carrier proteins are saturated
Transport maximum determines the renal threshold, or the plasma concentration at which a specific compound or ion will begin appearing in the urine
For example, renal threshold
for glucose is about 220 mg/dL
Untreated diabetes mellitus
may be 400 mg/dL
Nephron Loop
Thin, descending limb = water permeable
Reabsorbs about 15% of water, now about 80% of total
Thick, ascending limb = water impermeable
Reabsorbs about 25% of Na+ and Cl-, now about 90% of total
So tubular fluid becomes more dilute
Leaves excess Na+ and Cl- in peritubular fluid (interstitial fluid around tubules)
Osmolarity
Osmolarity = amount of dissolved particles in one liter of solution
Normal range of physiological osmolarity is measured in milliosmoles per liter (mOsm/L)
Blood plasma, interstitial fluid, and
intracellular fluid (cytosol) measure about 300 mOsm/L
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Countercurrent Multiplier= establishes salinity gradient
in renal medulla
Distal Convoluted Tubule (DCT) & Collecting Duct
Fluid arriving at DCT contains about 20% of water & 10% of salts (mostly NaCl)
Reabsorption of remaining water and salts is variable and under the control of hormones
This allows for the regulation of water and salt balances
Aldosterone = “salt-retaining hormone”
Secreted by adrenal cortex, triggered by;
• Low blood Na+
• Elevated K+
• Low BP Renin Angiotensin II
Principle cells with aldosterone receptors in
• Ascending limb of nephron loop
• Distal convoluted tubule
• Collecting duct (cortical part only)
Aldosterone binds nuclear receptors & activates transcription of gene for Na+/K+ pump
In 10 to 30 minutes effect of pumps at work in membrane is seen
Na+/K+ pump in tubular cell basal membrane
Moves Na+ out = Na+ reabsorptionMoves K+ in= K+ secretion
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Atrial Natriuretic Peptide (ANP)
Secreted by atrial myocardium in response to high blood pressure
Dilates afferent & constricts efferent arterioles• Inhibits renin and aldosterone secretion• Inhibits ADH secretion and action of ADH on
kidneys• Inhibits NaCl reabsorption by collecting ducts
Reduces blood volume
which reduces blood pressure
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Urine Formation
1) Glomerular Filtration
2) Tubular Reabsorption
& Secretion
3) Water Reabsorption
(Conservation)
Antidiuretic Hormone (ADH)
Secreted by posterior pituitary, triggered by;
Dehydration & rising blood osmolarity
Causes more water to be reabsorbed in collecting duct, so less water lost in urine
Relies on high osmolarity in extracellular fluid of inner medulla
Total = 1200 mOsm/L
• About 750 mOsm/L from NaCl
• About 450 from urea (papillary region)
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ADH causes cells in collecting duct to synthesize aquaporins (water-channel proteins) & install them in plasma membrane
Resulting in more water being able to pass through the tubule wall
What determines the highest osmolarity possible for urine?
Renal ClearanceRenal Clearance = volume of blood plasma from
which a particular waste is completely removed in one minute
Glomerular filtration of waste (125 ml/min)
PLUS amount added by tubular secreation
MINUS amount removed by tubular reabsorption
Renal Clearance
Inulin = 125 ml/min
Urea = 60 ml/min
Creatinine = 140 ml/min
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Conduction of Urine
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Conduction of Urine
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